docs(core): rewrite flush, WallClockStore and age_ms rustdoc from the spec

- Store::flush and Node::flush: the §4 contract (durability point, time
  anchor, harmless twice, earlier now clamped, Err(Backend) leaves the
  store usable, application decides when). No "infallible" or
  "duplicate custodian" wording.
- WallClockStore and Node::set_wall_clock / peer_time_estimate: define
  quarantine generically, a second set_wall_clock is a no-op returning
  the first report, peer time is unauthenticated until 1.1.
- Observation::age_ms: a lower bound that every holder advances; use
  observed_at for absolute time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tines-core/src/node.rs

@@ -219,8 +219,13 @@ impl<S: Store, T: Transport> Node<S, T> {
         &self.store
     }
 
-    /// Lift the quarantine on held observations once the host has trusted wall
+    /// Pass trusted wall time (ms since the Unix epoch) to a
-    /// time, by folding the wall clock into a [`WallClockStore`] backend.
+    /// [`WallClockStore`] backend, which folds the downtime into what it held
+    /// at start-up and lifts its quarantine (see [`WallClockStore`]).
+    ///
+    /// Call it once, when the host first has trusted time. A second call is a
+    /// no-op that returns the first fold's report. Core forwards the value and
+    /// never reads it.
     pub fn set_wall_clock(&mut self, wall_now: u64, now: Tick) -> <S as WallClockStore>::FoldReport
     where
         S: WallClockStore,
@@ -228,8 +233,11 @@ impl<S: Store, T: Transport> Node<S, T> {
         self.store.set_wall_clock(wall_now, now)
     }
 
-    /// Best estimate of trusted wall time from a peer's observation, if the
+    /// The backend's estimate of the current wall time from peers'
-    /// backend supports it (see [`WallClockStore`]).
+    /// observations, if any (see [`WallClockStore::peer_time_estimate`]).
+    ///
+    /// Peer time is unauthenticated until 1.1: bound the estimate before
+    /// passing it to [`Self::set_wall_clock`].
     pub fn peer_time_estimate(&self, now: Tick) -> Option<u64>
     where
         S: WallClockStore,
@@ -237,13 +245,16 @@ impl<S: Store, T: Transport> Node<S, T> {
         self.store.peer_time_estimate(now)
     }
 
-    /// Best-effort durability flush, advancing each stored observation's age
+    /// Flush the store: its durability point (see [`Store::flush`]).
-    /// up to `now`.
     ///
-    /// This is the durability boundary a host reaches on its flush schedule —
+    /// Everything inserted, released or trimmed so far becomes durable, and
-    /// delegates to [`Store::flush`], so it is infallible by contract: a failed
+    /// `now` becomes the time anchor for ages after a restart. Flushing twice
-    /// flush is, at worst, a duplicate custodian on the next exchange, never a
+    /// is harmless, and a `now` earlier than the last anchor is clamped to it.
-    /// lost message.
+    /// A failure returns [`StoreError::Backend`] and leaves the node and its
+    /// store usable, so the application can retry on its next flush.
+    ///
+    /// Core never flushes on its own: the application decides how often,
+    /// like `poll` and `trim`. On flash, every flush costs wear.
     pub fn flush(&mut self, now: Tick) -> Result<(), StoreError> {
         self.store.flush(now)
     }

tines-core/src/observation.rs

@@ -46,13 +46,17 @@ pub struct Observation {
     pub observer: NodeId,
     /// The observer's per-observation sequence number.
     pub seq: u64,
-    /// The observer's elapsed time for this observation, in ms.
+    /// How long ago the observation was made, in ms: a *lower bound* on its
+    /// true age.
     ///
-    /// This is a *lower bound* on the observation's true age: it counts time
+    /// Every holder advances it by the time the observation spends in its
-    /// only while the observer's clock runs. Time the host spent powered off is
+    /// store, so it counts the time spent on running clocks. It misses time a
-    /// recovered when the store is opened (see [`Self::observed_at`]); the time
+    /// holder was powered off until the holder's store recovers it from a
-    /// in an un-flushed crash window is bounded by the host's flush cadence,
+    /// trusted wall clock, time between a holder's last flush and a crash, and
-    /// and in-flight time is negligible. Relays never advance it.
+    /// time in flight. Every error makes an observation look younger, never
+    /// older.
+    ///
+    /// For absolute time, use [`Self::observed_at`].
     pub age_ms: u64,
     /// The observer's wall-clock time, in ms since the Unix epoch (UTC), when
     /// this observation was published, or `None` if the observer has no

tines-core/src/store.rs

@@ -107,14 +107,27 @@ pub trait Store {
     /// re-inserting them later is accepted.
     fn trim(&mut self, policy: &TrimPolicy, now: Tick) -> usize;
 
-    /// Best-effort durability flush, advancing each stored observation's age
+    /// The store's durability point.
-    /// up to the caller's `now` and recording `now` as the flush clock.
     ///
-    /// Flush must not lose messages, must never resurrect a released message,
+    /// A flush does two things:
-    /// and is infallible by contract: a failed flush is, at worst, a
+    /// - everything inserted, released or trimmed so far becomes durable;
-    /// duplicate custodian on the next exchange — never a lost message.
+    /// - `now` becomes the time anchor for ages after a restart, so a
-    /// Backends that cannot persist more cheaply default to a no-op that
+    ///   persistent backend can count the time between this flush and the
-    /// simply returns `Ok(())`.
+    ///   restart into each age.
+    ///
+    /// More of the contract:
+    /// - Flushing twice is harmless.
+    /// - A `now` earlier than the last anchor is clamped to it.
+    /// - A flush doesn't change any age [`Self::get`] reports at `now` or
+    ///   later, evict anything, or undo a release.
+    /// - A failure returns [`StoreError::Backend`] and leaves the store
+    ///   usable. What wasn't made durable is at risk only if the process
+    ///   then stops before a later flush succeeds.
+    /// - The application decides when to flush; core never calls `flush`
+    ///   itself. On flash, every flush costs wear.
+    ///
+    /// The default does nothing and returns `Ok(())`, which is correct for a
+    /// store that holds nothing across a restart, such as `MemStore`.
     fn flush(&mut self, now: Tick) -> Result<(), StoreError> {
         let _ = now;
         Ok(())
@@ -128,24 +141,38 @@ pub(crate) fn current_age(obs: &Observation, stored_at: Tick, now: Tick) -> u64
     obs.age_ms.saturating_add(now.saturating_sub(stored_at))
 }
 
-/// A [`Store`] that a host can feed trusted wall-clock time into.
+/// A [`Store`] that holds data across power-off and can take trusted wall
+/// time from the host (spec §9, decision H).
 ///
-/// tines-core itself never deals with wall time — this trait is a host-facing
+/// A store that has been powered off can't tell how long it was off from
-/// hook so a host can lift the quarantine on observations held across a
+/// ticks alone, which restart on every boot. A backend that implements this
-/// power-off, and estimate trusted time from a peer's observation, without a
+/// trait holds back what it loaded at start-up (its *quarantine*: not
-/// general `&mut Store` backdoor (decision F in the spec). Only backends that
+/// offered, delivered or acked, but still deduped) until the host has
-/// support a wall-clock fold implement it; `MemStore` and other pure-tick
+/// trusted wall time, then folds the downtime into those ages. Core never
-/// stores do not.
+/// reads or interprets a wall time: this trait only carries the host's value
+/// to the backend, and the backend's estimate back to the host. Hosts reach
+/// it through [`Node::set_wall_clock`](crate::Node::set_wall_clock) and
+/// [`Node::peer_time_estimate`](crate::Node::peer_time_estimate), so no
+/// `&mut Store` escapes the facade. Only backends that hold data across
+/// power-off implement it; `MemStore` doesn't.
 pub trait WallClockStore: Store {
-    /// Summary of a wall-clock fold, for the host's logs.
+    /// The backend's summary of what a fold did, for the host's logs.
     type FoldReport;
 
-    /// Fold trusted wall time into the store, lifting the quarantine on held
+    /// Fold trusted wall time into the store: `wall_now` is the host's
-    /// observations. The host calls this exactly once, on the first trusted
+    /// trusted wall time in ms since the Unix epoch, and `now` the current
-    /// time.
+    /// tick. Lifts the quarantine on what the store held at start-up.
+    ///
+    /// The fold runs once, when the host first has trusted time. A second
+    /// call is a no-op that returns the first fold's report.
     fn set_wall_clock(&mut self, wall_now: u64, now: Tick) -> Self::FoldReport;
 
-    /// Best estimate of trusted wall time from a peer's observation stored
+    /// An estimate of the current wall time (ms since the Unix epoch) from
-    /// since the last open, if any. `None` means stay quarantined.
+    /// peers' observations stored since start-up that carry `observed_at`,
+    /// or `None` if there are none yet.
+    ///
+    /// Peer time is unauthenticated until 1.1: a peer can claim any
+    /// `observed_at`, so the host bounds the estimate before it passes it to
+    /// [`Self::set_wall_clock`].
     fn peer_time_estimate(&self, now: Tick) -> Option<u64>;
 }